[CMake] CMake, Mingw-w64 32 bit exception handling.

[Robert Maynard]

A quick scan of CMake source code shows that we don't have any
references to gcc_eh anywhere. I way this could be occurring is
through CMake detection of the implicit libraries that a compiler
requires for each language. In particular it could be that C code for
mingw by default uses gcc_eh while C++ doesn't. The culprit could also
be a FindPackage* you are using.

On Wed, Aug 23, 2017 at 4:55 AM, Arne Kjetil Andersen <oelbox at gmail.com> wrote:
> Greetings.
>
> I'm a developer on a fairly large project where I'm using CMake version
> 3.9.1
>
> I primarily work on linux, but also cross compiles for windows using
> Mingw-w64 on my linux box.
>
> I have encountered an issue which I'm having some trouble figuring out.
> Running through some of my tests where an exception is thrown (on purpose)
> the 32 bit version compiled with Mingw-w64-g++ version 7.1.1 just calls
> terminate even though there are try catch blocks. Now mind you, this all
> works fine on the native linux compiled version of my tests, and also the 64
> bit windows version compiled with Mingw-w64-g++ version 7.1.1.
>
> Going through all the projects CMakeLists.txt I could not find any reason
> for this behavior, but tried to add -fexceptions as a compiler option in the
> top most CMakeLists.txt file for the 32 bit mingw-w64 compiler.
> Unfortunately this made no difference.
>
> So investigating some more I took a look at the linklibs.rsp file generated
> for that particular test executable, and noticed this entry:
> -lgcc_eh -lgcc_eh
>
> (yes it's twice, but that is not the issue, although that might be a cmake
> bug?).
> (also note - this option is also present for the 64 bit build files for
> mingw-w64, but there it works as expected).
>
> Now, removing those two library link options from the linklibs.rsp file
> makes the 32 bit windows version of test application work as expected. I am
> not sure what libgcc_eh.a actually does (tried searching for some
> information, but had little luck actually figuring that out), but clearly it
> has something to do with exception handling.
>
> Now I figured I would create a small minimal example that would reproduce
> this issue outside my projects source tree. So basically created a small
> program that throws an exception, and catches that. Created a CMakeLists.txt
> file with the same general options as my farily large project, and had cmake
> generate the build files for 32 bit mingw-w64. Inspecting the linklibs.rsp
> file I was surprised to see that "-lgcc_eh" were nowhere to be found, and as
> such the 32 bit version of this test worked fine.
>
> So, my question is, does anyone know under which circumstances cmake will
> add -lgcc_eh to linklibs.rsp, and is there any way I can prevent cmake from
> doing so for the 32 bit mingw-w64 compiler?
>
> Also, maybe I'm going about this issue the wrong way, and that my findings
> mentioned above is not a good way of handling this. Or maybe this might be a
> bug with the 32 bit mingw-w64 compiler?
>
> I should probably also mention that the 32 bit version of Mingw-w64 uses the
> sjlj exception handling mechanism.
>
> Any help and pointers would be greatly appreciated - cause adding a step in
> the developer documentation to go into the linklibs.rsp file to remove
> -lgcc_eh is kind of a last resort.
>
> Thanks for any input on this matter, and please let me know if attaching
> CMakeOutput.log or other files would be beneficial.
>
> Best Regards,
> Arne Kjetil Andersen
>
>
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